In an increasingly mobile society, portable electric appliances are playing an ever greater role. For this reason, batteries, in particular rechargeable batteries (known as secondary batteries or accumulators), have for many years been used in virtually all aspects of life. Secondary batteries now have to meet a complex requirement profile in terms of their electrical and mechanical properties. Thus, the electronics industry is demanding new, small, lightweight secondary cells or batteries having a high capacity and a high cycling stability in order to achieve a long life. Furthermore, the temperature sensitivity and the spontaneous discharge rate should be low in order to ensure high reliability and efficiency. A high measure of safety in use is required at the same time. Lithium ion secondary batteries having these properties are also of interest for, in particular, the automobile sector and could, for example, in future be used as power stores in electrically operated vehicles or hybrid vehicles. In addition, batteries which have advantageous electrokinetic properties are required here in order to be able to achieve high current densities. In the development of new types of battery systems, being able to produce rechargeable batteries in an inexpensive way is also of particular interest. Environmental aspects are also playing an increasing role in the development of new battery systems.
The anode of a modern high-energy lithium battery at present typically comprises graphite but can also be based on metallic lithium, a lithium alloy or lithium-metal oxides. The use of lithium-cobalt oxides has been found to be useful in recent years for construction of the cathode of a modern lithium battery. The two electrodes in a lithium battery are connected to one another using a liquid or solid electrolyte. During (re)charging of a lithium battery, the cathode material is oxidized (e.g. according to the equation: LiCoO2→n Li++Li(1-n)CoO2+ne−). In this way, the lithium is liberated from the cathode material and migrates in the form of lithium ions to the anode where the lithium ions are bound with reduction of the anode material; in the case of graphite, intercalated as lithium ions with reduction of the graphite. Here, the lithium occupies the sites between the layers in the graphite structure. On discharge of the battery, the lithium bound in the anode is released from the anode in the form of lithium ions and oxidation of the anode material takes place. The lithium ions migrate through the electrolyte to the cathode and are bound there with reduction of the cathode material. Both during discharge of the battery and during recharging of the battery, the lithium ions migrate through the separator.
However, a significant disadvantage of the use of graphite in Li ion batteries is the comparatively low specific capacity with a theoretical upper limit of 0.372 Ah/g. Graphite-like carbon materials other than graphite, for example carbon black such as acetylene black, lamp black, furnace black, flame black, cracking black, channel black or thermal black, and also glossy carbon or hard carbon, also have similar properties. In addition, such anode materials are not unproblematical in terms of their safety.
Higher specific capacities can be obtained when lithium alloys such as LixSi, LixPb, LixSn, LixAl or LixSb alloys are used. In this case charge capacities up to 10 times the charge capacity of graphite are possible (LixSi alloy, see R. A. Huggins, Proceedings of the Electrochemical society 87-1, 1987, pp. 356-64). A significant disadvantage of such alloys is the dimensional change which they undergo during charging/discharge, which leads to disintegration of the anode material. The resulting increase in the specific surface area of the anode material results in capacity decreases due to irreversible reaction of the anode material with the electrolyte and increased sensitivity of the cell to heat, which in an extreme case can lead to strongly exothermic destruction of the cell and is a safety risk.
The use of lithium as electrode material is problematical for safety reasons. In particular, the deposition of lithium during charging results in formation of lithium dendrites on the anode material. These can lead to a short circuit in the cell and can in this way bring about uncontrolled destruction of the cell.
EP 692 833 describes a carbon-comprising insertion compound which comprises carbon together with a metal or semimetal which forms alloys with lithium, especially silicon. The compound is prepared by pyrolysis of polymers comprising the metal or semimetal and hydrocarbon groups, e.g. in the case of silicon-comprising insertion compounds, by pyrolysis of polysiloxanes. The pyrolysis requires drastic conditions under which the primary polymers are firstly decomposed and carbon and (semi)metal and/or (semi)metal oxide domains are subsequently formed. The preparation of such materials generally leads to poorly reproducible qualities, presumably because the domain structure is impossible or difficult to control due to the high energy input.
H. Tamai et al, J. Materials Science Letters, 19 (2000) pp. 53-56, propose silicon-doped carbon-comprising materials which are obtained by pyrolysis of pitch in the presence of polysiloxanes as anode material for Li ion batteries. For the abovementioned reasons, these materials have disadvantages comparable to those of the materials known from EP 692 833.
US 2002/0164479 describes a particulate carbon-comprising material as anode material for Li ion secondary cells, where the particles of the carbon-comprising material comprise graphite particles on whose surface a plurality of “complex particles” are located and enveloped by an amorphous carbon layer. The complex particles are in turn made up of a particulate, crystalline silicon phase, particles of conductive carbon located thereon and a carbon envelope. The complex particles have particle sizes in the range from 50 nm to 2 μm and the graphite particles have particle sizes in the range from 2 to 70 μm. To produce the materials, complex particles are firstly produced by carbonization of a mixture of phenolic resin, silicon particles and conductive carbon black and these are subsequently mixed with particulate graphite and further phenolic resin and carbonized. Not least because of the double carbonization, the production of these materials is comparatively complicated and leads to poorly reproducible results.
US 2004/0115535 describes a particulate carbon material as anode material in Li ion secondary batteries, in which silicon particles having dimensions of less than 100 nm are dispersed together with SiO2 particles in particles of a continuous carbon phase. To produce such materials, a mixture of SiOx particles (0.8≦x≦1.5), carbon particles and a carbonizable substance is carbonized at elevated temperature. The use of SiOx particles makes the process complicated.
H. H. Kung et al., Chem. Mater., 21 (2009) pp. 6-8, describe silicon nanoparticles which have particle sizes of <30 nm and are embedded in a porous carbon matrix which is bound covalently and to the silicon particles and their suitability as elektroactive anode materials in Li ion batteries. They are produced by reaction of hydrogen-terminated silicon nanoparticles with allylphenol in a hydrosilylation reaction, subsequent reaction of the hydrosilylated particles with formaldehyde and resorcinol to form a formaldehyde resin which is covalently bound to the nanoparticles and carbonization of the material obtained. Not least because of the use of H-terminated Si nanoparticles and the hydrosilylation, the production of these materials is comparatively complicated and expensive. In addition, this process leads to poorly reproducible results, presumably because the reaction leads to incomplete and variable conversions due to the disadvantages inherent in a surface reaction, e.g. steric hindrance and diffusion phenomena.
I. Honma et al., Nano Lett., 9 (2009), describe nanoporous materials which are made up of SnO2 nanoparticles embedded between exfoliated graphite layers. These materials are suitable as anode materials for Li ion batteries. They are produced by mixing exfoliated graphite layers with SnO2 nanoparticles in ethylene glycol. The exfoliated graphite layers were in turn produced by reduction of oxidized and exfoliated graphite. This process, too, is comparatively complicated and suffers in principle from similar problems as the process described by H. H. Kung et al.
In summary, it may be said that the carbon-based or lithium alloy-based anode materials which have hitherto been known from the prior art are unsatisfactory in respect of the specific capacity, the charging/discharge kinetics and/or the cycling stability, e.g. decrease in the capacity and/or high or increasing impedance after a plurality of charging/discharge cycles. The composite materials having a particulate semimetal or metal phase and one or more carbon phases which have recently been proposed in order to solve these problems are able to solve these problems only partly, with the quality of such composite materials generally not being reproducible. In addition, production of these materials is generally so complicated that economical use is not possible.